The present invention relates to a process of producing a thin film transistor in an active matrix type liquid crystal display apparatus using a thin film transistor.
Recently, a display device employing liquid crystal has been widely used. Therefore, miniaturization and low power consumption of a liquid crystal display apparatus are further required. Together with this, a method of integrally forming a thin film transistor that serves as a switching element to be used as a drive element on a glass substrate of a liquid crystal panel has been used in practice. As a method of forming a semiconductor region of a thin film transistor, a method using polycrystalline silicon is used.
As for a method of forming a thin film transistor, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-148834, a channel separating region (11) is first formed on a P-type Si substrate (10) and a gate electrode (13) is formed in a region sectioned by the channel separating region (11) via a gate oxide film (12) (FIG. 1a). Formation of the gate electrode (13) can be carried out by forming a Poly-Si film of a prescribed pattern on the Si substrate (10), forming a resist film (14) of a prescribed pattern, and removing by etching the Poly-Si film while using the resist film (14) as a mask, similarly to the prior art. At this time, the length of the gate electrode (13) is formed to be greater than the final length in a prescribed amount, i.e. an amount corresponding to a side wall (6) as shown in FIG. 2.
Next, a resist film (15) for covering regions other than those that become a source and a drain, for example, regions of the channel separating region (11), the P-channel type MOS transistor, etc. is formed without removing the resist film (14) on the gate electrode (13), and N.sup.+ -type regions (16S) (16D) are formed by implanting highly concentrated N-type impurity, for example, arsenic (As.sup.+) while using these resist films (14) (15) as masks (FIG. 1b). The N.sup.+ -type regions (16S) (16D) respectively become a source region and a drain region.
Next, the resist films (14) (15) are isotropically removed in a prescribed amount by the plasma treatment including oxygen (FIG. 1c). In this plasma treatment, the resist film (14) is removed in an amount corresponding to the size of the gate electrode (13) that is formed to be larger than necessary. That is, part of the resist film (14) is removed so that the size of the resist film (14) left after the plasma treatment can be that of a final gate electrode (13'). Further, the gate electrode (13) is etched by using a resist film (14') in a prescribed size as a mask, both sides of the gate electrode (13) are removed, and N-type impurity, for example, P.sup.+ is implanted at a concentration lower than that when the N.sup.+ -type regions (16S) (16D) are formed while using the resist films (14') (15') as masks to form N.sup.- -type diffusion regions (17S) (17D).
However, according to the thin film transistor producing method described in Jpn. Pat. Appln. KOKAI Publication No. 3-148834, the width of the LDD (Lightly Doped Drain) is determined according to the lithography, and therefore, the widths of the LDDs formed at both left and right sides of the gate are different or vary from substrate to substrate because of warp of the glass substrate having Si or an error in positioning the mask. This can make the magnitude of the ON current of the thin film transistor irregular, and therefore, the yield and reliability of the products cannot be improved. As a result, the producing cost of the liquid crystal display apparatus may be increased.